When Microsoft got fined $732 million by the European Commission on Wednesday for reneging on an agreement to give Windows users a clearer choice of browser, an interesting question in the back of many people’s minds was how on earth no-one noticed the browser choice screen’s omission for more than a year.

After all, Windows 7 Service Pack 1 came out in February 2011 and it was only in mid-2012 that the Commission woke up and realized the fact that it was missing the screen in question. Microsoft claimed it was itself unaware of the omission until the Commission brought it up.

Well, we’re still fuzzy on how both Microsoft and the Commission stayed oblivious for so long, but we do now have a better idea of how the company got busted: according to anonymous sources quoted in a Financial Times piece (registration required) that went up a few hours ago, the tip-off came from Google and Opera.

Opera is certainly no surprise – the Norwegian browser vendor was after all the original complainant that led the Commission to wring promises of good behavior from Microsoft back in 2009. But Google’s involvement, if the report is correct, shows this to be just one stage in an increasingly bitter war.

Remember that Chrome vendor Google is alsoembroiled in an EU antitrust investigation, in this case regarding allegations of search result manipulation and various other anticompetitive practices. And who kicked off that investigation? Why, Microsoft of course, along with various others subsequently involved in its faux-grassroots “Fair Search” organization.

Then we have Microsoft’s recent Scroogled anti-Google smear campaign, and Google’s decision to suddenly yank Exchange ActiveSync support for Windows Phone owners who use Gmail services, which threatened a serious hit on that smartphone platform’s usability.

In isolation, each one of these moves can be explained and perhaps justified on its own merits. Together, though, they paint a picture of escalating nastiness. Sure, companies fight all the time — it’s part of healthy competition. But right now the goings-on between Google and Microsoft risk appearing as petty and destructive as those between Apple and Samsung.

This article was updated at 5:45am PT to make it more explicit that Google makes the Chrome browser and therefore has an interest in browser choice.